Helium occurs naturally in very low concentrations within underground natural gas and carbon dioxide reservoirs. In some instances, helium is present at sufficiently high concentrations to justify its recovery. In general, helium can be effectively recovered from gas streams containing at least 0.1 mole percent helium or greater. In such recovery methods, helium is first concentrated into a crude helium stream that contains approximately 70 mole percent helium. The crude helium may then be stored, typically in an underground reservoir, or substantially further purified and liquefied for merchant sale.
There is a growing demand for large quantities of carbon dioxide in applications such as enhanced oil recovery. This demand has increased the interest of extracting valuable helium from such carbon dioxide-rich streams. It is to be noted that vast quantities of carbon dioxide are processed in enhanced oil recovery applications, normally greater than 10,000 tons per day. The high unit value of helium can substantially improve overall project economics.
In the prior art, natural gas-hydrocarbon streams have been subjected to helium extraction. For instance, in U.S. Pat. No. 3,355,902 a gas stream that has an extremely high hydrocarbon content and a low helium and carbon dioxide content is cooled and then introduced into a fractionator to produce a column overhead stream that contains dissolved helium. Boil up is produced within the fractionator by heating the liquid column bottoms. The helium rich overhead is further cooled, flashed and separated in several stages to provide a crude helium product.
U.S. Pat. No. 5,329,775 discloses a helium production system for separating helium from a stream containing helium, hydrocarbons and carbon dioxide. Again the helium and carbon dioxide is present within a feed in very low concentrations. The feed is rectified within a liquid-vapor contact column to produce a tower overhead which is further cooled and phase separated to produce the helium containing stream.
Both of the patents mentioned above are not applicable to the recovery of helium from carbon dioxide containing streams obtained from known underground reservoirs in which carbon dioxide is present at a concentration, greater than about 30 mole percent. The low temperature processes illustrated in such patents would tend to freeze carbon dioxide and therefore would be ineffective. In both of these patents only a fraction of the feed is liquefied prior to the trace carbon dioxide removal step.
As will be discussed, the present invention provides a method of separating helium from a gaseous carbon dioxide containing feed that contains at least 30 mole percent in an energy efficient manner. Such method allows the carbon dioxide containing fraction to be repressurized and returned for use or storage at high pressure. In case of feed streams having a high content of light components, such as nitrogen, the recompression can be carried out in an energy efficient manner. With respect to the separated light components, the present invention is intended to be used with further known purification techniques to produce a crude helium stream that can be stored or further processed. As will be discussed, such method is also applicable to separating other light components such as hydrogen and neon from feed streams having a similarly high carbon dioxide content.